1. Field of the Invention
The invention relates to thermal image detectors, notably pyroelectric detectors, designed to produce an image of a scene in infrared light, and especially to detectors that work at ambient temperature.
Pyroelectric detectors absorb infrared radiation to heat a pyroelectric layer, namely a layer with the property of generating surface charges as a function of the temperature. The charges generated, converted into voltage, are processed to give a measurement of the intensity of infrared radiation received by the detector. There may be other types of infrared image detectors relying on effects other than the pyroelectric effect but relying also on the heating of a layer. These other types are also concerned by the invention which, however, shall be described in detail solely with reference to a pyroelectric detector.
The detector may be a point detector, or it may comprise a column of pyrosensitive points to make an infrared linear image, or again it may be a matrix network of points to form a 2D image. In particular, this image may be an image of the temperature distributions of an observed scene.
There are hybrid detectors using two substrates: one substrate that fulfils the pyroelectric function and one substrate that fulfils the signal processing functions. The two substrates are bonded face to face to connect each pyrosensitive point of the first substrate to a point of the second substrate.
More recently, monolithic detectors have also been proposed. These monolithic detectors are constituted by an integrated circuit substrate covered with pyroelectric material (a pyroelectric polymer material) that can be deposited in a thin layer. The substrate bears the circuit elements needed for the processing of the pyroelectric signal that is generated.
2. Description of the Prior Art
Since the pyroelectric material produces charges proportionally to its heating, and since this heating is an integral of the intensity of radiation received, the pyroelectric detector must work differentially and not absolutely, especially if fixed scenes are to be observed. Indeed, a constant intensity of radiation, representing the luminance of a point emitting an infrared radiation, will prompt a gradual heating of the material up to a saturation value that cannot be used to deduce the intensity of the radiation received. Furthermore, an absolute measurement of temperature would depend excessively on the variations in ambient temperature of the detector and would not be sufficiently representative of the temperature distributions of the scene observed.
This is why, it is provided that the detector will be alternately illuminated, i.e. subjected to infrared radiation, and then masked. The period of the illumination/masking alternation should be sufficient to give the pyroelectric material the time to be heated during the illumination and the time to be cooled during the masking. The period is equal, to, for example 1 to 50 Hertz (10 milliseconds of illumination for 10 milliseconds of masking). What is measured then is not the mean heating but the amplitude of variation of the heating during the alternation. This amplitude represents the intensity received, and works much better than the mean heating, which depends on too many other parameters.
The curve of FIG. 1 shows the evolution of the temperature of the pyroelectric layer when the illumination is thus alternated. The curve is expressed directly in terms of voltage as a function of time, the voltage indicated being a fictitious voltage that represents an output signal of the detector, it being assumed that this signal is proportional to the heating of the pyroelectric material.
The temperature rises at the start of an illumination phase and tends towards a high saturation value that depends not only on the infrared intensity received but also on the heat losses of the pyroelectric layer. Then, it falls again as soon as the masking phase starts and tends also towards a low saturation value, with a speed that depends also on the thermal losses. The difference between the voltage at the end of the illumination phase and the voltage at the end of the masking phase gives a good measurement of the intensity of the infrared radiation received.
The detection consists then, broadly speaking, in measuring a sample of a signal VSH at the end of an illumination phase and a sample of a signal VSB at the end of a masking phase, and in taking the difference VSH-VSB, to deduce therefrom a value of infrared intensity received.
It will be understood that the period of the illumination/masking alternation should be such that the pyroelectric layer has enough time to be heated and cooled at each phase, so that the variations of charges generated are sufficient in amplitude. If the duration of each phase is too short, then the signal-to-noise ratio is too small, the noise considered here being a noise that is independent of the duration of the phase. If it is too long, the gain in terms of signal level is no longer improved owing to the saturation of the signal curve (see FIG. 1). Furthermore, for reasons of compatibility with standard imaging systems, it is sought to put out an image at a frequency of 25 to 60 Hz.
These reasons most usually mean that the shutter has to be made to work at a frequency that corresponds precisely to the output frequency of the images (25, 30, 50 or 60 Hz) for which the signal level obtained is sufficient without being excessively close to saturation.
However, it has been observed that certain structures of pyroelectric detectors are sensitive to a noise whose amplitude is proportional to the duration of the integration. Without going into every case where this is possible, it may be pointed out, by way of an example, that this is the case in a monolithic detector structure wherein each pixel is defined by a is pyroelectric capacitor and wherein the capacitor connected to a semiconductor substrate by means of a reverse-biased diode. The natural thermal generation of the charge carriers in this diode disturbs the pyroelectric charge generated by the observed image. And this disturbance is proportional to the duration of each phase, a resetting-to-level operation being done at the end of each phase to prevent a permanent drift.
It can be the case that this noise is not negligible for the durations of phases considered here above.
The invention proposes a method to increase the signal-to-noise ratio of the thermal detectors.